This invention relates to rotatable, hollow shafts and more particularly to means for making temperature measurements at any point within the bore of the hollow shaft during shaft rotation.
Hollow shafts, when rotated, are subjected to centrifugal forces which cause stresses in the shaft which are highest near the shaft bore. Such hollow shafts are commonly used in machines such as steam turbines which often operate at elevated temperatures. The exterior of those shafts are commonly directly subjected to those elevated temperatures.
Shaft material temperature is substantially constant from its radially outer to its radially inner surface after the shaft's outer surface has been exposed to elevated temperatures for an extended period of time. During startup of the steam turbine or other device, however, the shaft's material temperature must be increased from near ambient to operating temperatures approaching 1,000.degree. F. Commonly used startup procedures include gradually increasing the rotational velocity and turbine temperature by slowly incrementing motive fluid (steam) flow through the turbine and along the shaft's exterior. On initiating steam flow through the turbine the shaft's exterior surface temperature will be substantially equal to the motive fluid temperature while the hollow shaft's bore temperature will approach ambient as the turbine's shutdown time increases. Such temperature differences across the shaft's radial thickness promote thermal stresses in the shaft. Such thermal stresses when added to the stresses imposed by centrifugal loading of the shaft can, if left unchecked, result in mechanical failure. Temperature differentials and accompanying thermal stresses vary also with axial location within the turbine since steam temperatures decrease during the steam's axial traversal through the turbine. Knowledge of the inner shaft surface's temperature is highly desirable if optimal motive steam flow rates and temperatures are to be controlled during turbine startup so as to simultaneously provide safe and rapid turbine startups.
In the past, it was common practice to measure the steam temperature at the shaft's surface and, with the material properties of the shaft, calculate the shaft's inner surface temperature. The fracture toughness of the turbine shaft has been found to be a good indicator of the mechanical properties of the shafts and is a function primarily of temperature with the lower temperatures tending toward lower toughness. Since the inner surface of the shaft is always at the lowest temperature in the shaft, during a heating transient, the shaft's toughness in the region of its bore is lowest and therefore more susceptible to fracture. While the former technique for determining bore temperature is satisfactory, a confirmation of such calculated temperatures is desirable for the purpose of providing greater confidence in such calculations and in permitting shorter turbine startup times due to the greater certainty of measuring the bore temperature.